The invention concerns devices and methods for imaging of a beam of particles composed of charged particles with a certain energy and angle distribution on a detector, and a spectrometer, especially for electron diffraction measurements, with energy and angle resolution.
When irradiating a material sample with electromagnetic radiation or charged particles, due to interaction in the sample, radiation of charged particles may happen, whose spatial (or angular) and energy distribution allow drawing of conclusions concerning physical or chemical processes in interaction or characteristics of the participating particle types or the interaction region. Numerous analysis methods have been developed on this basis, for instance using electron diffraction examinations or spectroscopic examinations.
A problem of the known analysis methods is simultaneous recording of angular and energy distribution of a particle beam (consisting e. g. electrons, ions or ion groups, atoms or atom groups). For instance, in the investigation of reflection of high energy electrons from a sample (RHEED method), there is interest in energy-filtered recorded diffraction images to be able to separate elastic and non-elastic scattering processes. Modeling of purely elastic scattering enables an improved structural resolution.
In DE-OS 197 01 192, conventional systems for simultaneous local and energy resolution for investigation of electron diffraction are explained. As a first embodiment, a scanning mechanism is implemented which enables scanning of sections of the diffraction image and its energy analysis are described, but is interference-prone and time-consuming, so that specifically realtime analyses, for instance for observation of surface changes on solids, are possible only in a limited manner or not at all. Real simultaneous energy and angle resolution is only achieved with a second embodiment, in which a diffraction pattern is viewed through pairs or triplets of filters (generally: groups of filtering electrodes). Predetermined deceleration fields are formed between the filtering electrodes, which for instance are passed by elastically scattered electrons and which reflect the non-elastic scattered electrons.
For realization of the second embodiment, for instance by Y. Horio, in xe2x80x9cJpn. J. Appl. Phys. xe2x80x9d, Vol. 35, 1996, p. 3559 ff., filtering electrodes in the form of sphere-shaped grids are described, but these are disadvantageous due to the necessarily small working distance, the small energy resolution and sensitivity with respect to contamination. In contrary to this, according to DE-OS 197 01 192 (see FIG. 16), improvement was achieved by use of filtering electrodes in the form of plane grid electrodes 3, 4 and the combination of such filtering electrodes with a deflection unit 1 for formation of parallel electron beams, whose mutual distance corresponds to angular distribution of the electrons and which are directed to the filtering electrodes. Using this combination of parallel electron paths with plane filtering electrodes, the above mentioned disadvantages of the technology according to Y. Horio with respect to the working distance of the detector 2 from the sample, the sensitivity and robustness of the imaging device can be overcome. For measuring tasks with extreme requirements, the imaging quality (highly resolving electron diffraction), for which for instance the distance of diffraction maximums must be measured with high precision, restricting image distortions occur also in case of the technology with plane filtering electrodes.
The image distortions for conventional energy-selective imaging are especially caused by inhomogenous scatter angle focusing and by interfering moirxc3xa9patterns at the grid-shaped filtering electrodes 3, 4. The limited quality of scatter angle focusing leads to the fact that especially in case of larger scatter angles the correspondence between the distance between the parallel partial beams and the angle distribution of the scattered electrons is no longer linear. Depending on the application, the image size and/or the resolution of the image is limited by this.
The object of the invention is to provide improved devices and methods for angle and energy resolving imaging of a particle beam, which are characterized by increased imaging quality, in particular by extended scattering light images and/or higher resolution of the images. It is furthermore the object of the invention to provide a correspondingly designed spectrometer and method for its operation and use.
A first important aspect of the invention is the provision of an imaging device for energy and angle selective imaging of charged particles, for instance electrons, with a preceding entrance aperture or a preceding entrance grid oriented transversely to the particle beam. The entrance grid is electrically insulated with respect to the deflection unit and is preferably at ground potential. It serves to limit the electric field caused by the deflection unit on the sample side and allows better edge beam control. Edge beam control means that the linearity of the correspondence between scattering angles and distances between the parallelized particle trajectories also in case of larger scattering angles up to the image edges is assured. According to a first embodiment, the entrance grid is a grid being essentially planar, standing vertically on the axis of the deflection unit, which is preferably adapted for imaging of larger scattering angle ranges of up to +/xe2x88x9210xc2x0. But if, depending on the application, the highest linearity (minimal distortion) is more important for imaging, then according to a second embodiment of the entrance grid, it is provided as a sphere-shaped grid. As a sphere-shaped grid, the entrance grid has the shape of a section of a sphere surface with an associated predetermined sphere radius.
Another important aspect of the invention consists in further improving a device according to the invention for energy and angle resolved imaging of a beam of charged particles on a detector device on the basis of the above mentioned combination of a deflection unit for creation of parallel and decelerated particle beams and a filtering device to the extent that the filtering device consists of a single filtering grid aligned vertically with respect to the parallel particle beams. This means that between the deflection unit and the detector device only one filtering grid electrode is located, extending transversely to the particle beams, and the space between the deflection unit and the detector device is otherwise free of further beam shaping electrodes. The filtering grid in association with the deflection unit forms a retarding field analyzer, which for instance serves to filter nonelastically scattered electrons for electron diffraction investigations.
The transition from a group of filtering electrodes to a single filtering grid forms an important advantage, especially in interaction with a preceding entrance grid, with respect to simplification of the overall installation as well as with respect to imaging quality. The inventor surprisingly found that using the filtering grid the same highly sensitive energy selectiveness may be achieved as using the conventional imaging device, but no moirxc3xa9patterns are created and therefore the imaging quality is improved.
The invention also relates to a spectrometer, in particular an electron diffraction spectrometer being provided with the imaging device described above. Preferred applications of such a spectrometer are in electron diffraction investigations for all usual energy levels (LEED, MEED, HEED, corresponding to low, medium, high energy electron diffraction, respectively), for energy and angle resolved ion scattering investigations, for structural analysis by elastic scattering and non-elastic diffusion (for instance on the basis of Kikushi lines) or for angle resolved electron spectroscopy. The use of the imaging device according to the invention is not limited to use in diffraction spectrometers. A device according to the invention may also be used as an electronic mirror for the so-called xe2x80x9ctime-of-flightxe2x80x9d spectroscopy.
The invention furthermore relates to a method for imaging of a particle beam consisting of charged particles with a specific energy and angle distribution on a detector unit for which the particles are shaped into parallel and decelerated particle beams or trajectories using a deflection unit and are directed to a filtering grid, which in interaction with the deflection unit lets particles pass energy selectively having an energy level above a certain cut-off energy to a detector and reflects particles at a lower energy level. According to a preferred embodiment of the invention, the particles before entrance into the deflection unit pass through a field-free space on the sample side, which is shielded against the deflection unit by a plane or sphere section shaped entrance grid.
The invention offers the advantages of extremely improved energy resolution, which for instance for 10 keV electron energy level amounts to about 2 eV. Furthermore, the imaging properties are substantially improved with respect to conventional devices, which is shown specifically in a reduction of distortion for imaging of diffraction maximums. Further advantages result from the simplified structure of the imaging device according to the invention, the freedom of the image from distortion (improved scatter angle focusing) and extension of imaging dimensions (enlargement of the scattering angles able to be imaged in evaluation).